The present invention relates to a method and an apparatus for the continuous production of rectangular foam blocks or sheets from a liquid froth or a foamable reaction mixture. Preferred froths and foamable reaction mixtures are those which react to form polyurethanes and typically consist of polyisocyanates, compounds containing isocyanate-reactive hydrogen atoms, such as polyester or polyether polyols, catalysts, stabilizers, and other additives. Typical blowing agents include water, low-boiling olefins, such as pentane, and liquid carbon dioxide.
In the continuous production of foam blocks or sheets, the foamable mixture, which generally has a viscosity of between 200 and 2,000 mPa.cndot.s, is applied to a lower facing sheet resting on a conveyor belt, distributed over the width of the conveyor belt, covered with an upper facing sheet, cured and cut into blocks transversely to the direction of movement of the conveyor belt.
The foamable mixture applied to the conveyor belt can have a density of between 50 and 1,200 kg/m.sup.3, depending on whether i) a bubble-free reaction mixture is used, which does not expand until applied to the conveyor belt as a result of the reaction of water with the isocyanate to form CO.sub.2 (chemical foam formation, rising time), or ii) a liquid froth is applied which has been produced by beating in air (mechanical foam formation), or iii) by adding liquid carbon dioxide under pressure (physical foam formation) and which then expands further by additional chemical foaming.
The most widely used industrial method for applying the foam comprises applying the foamable mixture or the liquid froth to the conveyor belt from one or more essentially spot-shaped discharge devices which are connected to the mixhead for the production of the reaction mixture. The problem posed by this method is not only that of distributing the foamable mixture evenly over the width of the conveyor belt, but also that of preventing the inclusion of air when the upper facing sheet is applied.
U.S. Pat. No. 4,108,585 (corresponding to German Auslegungschrift 2,557,572) proposes solving this problem by adding the upper facing sheet from above and deflecting it via a levelling device (described as a roller or rocker) which is positioned above the lower conveyor belt in such a manner that a pile of foamable mixture forms on the rear side (in relation to the direction of movement of the conveyor belt) which prevents air from entering beneath the upper conveyor belt and at the same time helps to level the distribution of the mixture over the width of the lower conveyor belt.
In order to ensure a uniform age of the foamable mixture, the conveyor belt is downwardly inclined in the direction of travel--at least in the region of application. This prevents the foamable mixture from spreading out in an opposite direction to the direction of travel at the actual point in time of its application, which would result in a disadvantageous age distribution of the foam. Due to this inclination of the conveyor belt, the foamable mixture is not only moved along by the lower conveyor belt but its rate of flow is also increased in relation to the conveyor belt. This rate of flow is greatest at the point on the downward slope of the lower belt furthest from the feed point in the direction of travel. Thus the foamable mixture passes beneath the levelling device which is equal in width to the lower conveyor belt, so that a rectangular foam cross-section with a uniform foam density cannot actually be obtained.
Furthermore, it has already been proposed in U.S. Pat. No. 4,264,291 (corresponding to European Patent 25,084) to use a flat elongated deflecting element for the upper facing sheet which is essentially parallel to the conveyor belt so that a flow channel is formed in the direction of the conveyor belt. It has, however, been found that a fixed distance between the flat deflecting element and the conveyor belt does not allow for possible minimal fluctuations in the flow behavior of the foamable mixture which occur when the unit is operated over longer periods. Thus the mixture is either pressed against the lateral boundary walls (if the distance is too small) or the mixture does not reach the lateral boundary walls (if the distance is too great). This is particularly the case where the foamable mixture already consists of a froth and has therefore been obtained by expanding a pressurized reaction mixture which, for example, contains physically dissolved carbon dioxide.